Forming Nitrogen-Containing Layers as Oxidation Blocking Layers

ABSTRACT

A method includes forming a silicon layer on a wafer, forming an oxide layer in contact with the silicon layer, and, after the oxide layer is formed, annealing the wafer in an environment comprising ammonia (NH3) to form a dielectric barrier layer between, and in contact with, the silicon layer and the oxide layer. The dielectric barrier layer comprises silicon and nitrogen.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.16/120,677, entitled “Forming Nitrogen-Containing Layers as OxidationBlocking Layers,” filed Sep. 4, 2018, which claims the benefit of U.S.Provisional Application No. 62/691,901, filed Jun. 29, 2018, andentitled “Forming Nitrogen-Containing Layers as Oxidation BlockingLayers,” which applications are hereby incorporated herein by reference.

BACKGROUND

Transistors are basic building elements in integrated circuits. Inprevious development of the integrated circuits, Fin Field-EffectTransistors (FinFETs) are formed to replace planar transistors. In theformation of FinFETs, semiconductor fins are formed, and dummy gates areformed on the semiconductor fins. Gate spacers are formed on thesidewalls of the dummy gate stacks. The dummy gate stacks are thenremoved to form trenches between the gate spacers. Replacement gates arethen formed in the trenches.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 16 illustrate the cross-sectional views and perspectiveviews of intermediate stages in the formation of a Fin Field-EffectTransistor (FinFET) in accordance with some embodiments.

FIGS. 17A and 17B illustrate cross-sectional views of FinFETs inaccordance with some embodiments.

FIG. 18 illustrates a process flow for forming a FinFET in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A Fin Field-Effect Transistor (FinFET) and the method of forming thesame are provided in accordance with some embodiments. The intermediatestages of forming the FinFET are illustrated in accordance with someembodiments. Some variations of some embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements. In accordancewith some embodiments of the present disclosure, silicon nitride monolayers are formed through annealing in ammonia. The silicon nitride monolayers are between oxide regions and semiconductor regions, so that thesilicon nitride mono layers may protect the semiconductor regions frombeing oxidized, especially in subsequent anneal processes, during whichoxygen may migrate to the semiconductor regions to cause the adverseoxidation of the semiconductor regions.

FIGS. 1 through 16 and FIGS. 17A and 17B illustrate the perspectiveviews of intermediate stages in the formation of a FinFET in accordancewith some embodiments of the present disclosure. The processes shown inFIGS. 1 through 16 and FIGS. 17A and 17B are also reflectedschematically in the process flow 200 as shown in FIG. 18.

FIG. 1 illustrates a perspective view of an initial structure. Theinitial structure includes wafer 10, which includes substrate 20.Substrate 20 may further include substrate (portion) 20-1. Substrate20-1 may be a semiconductor substrate, which may be a silicon substrate,a silicon germanium substrate, or a substrate formed of othersemiconductor materials. Substrate 20-1 may also be a bulk substrate ora semiconductor-on-insulator substrate.

In accordance with some embodiments of the present disclosure, theillustrated region is a p-type device region, in which a p-typetransistor such as a p-type Fin Field-Effect Transistor (FinFET) is tobe formed. Epitaxy semiconductor layer 20-2 may be epitaxially grown ontop of substrate 20-1 to form substrate 20. For example, epitaxysemiconductor layer 20-2 may be formed of silicon germanium (SiGe) orgermanium (without silicon therein). The germanium atomic percentage inepitaxy semiconductor layer 20-2 may be higher than the germanium atomicpercentage in substrate portion 20-1. In accordance with someembodiments of the present disclosure, the germanium atomic percentagein epitaxy semiconductor layer 20-2 (when formed of SiGe) is in therange between about 30 percent and 100 percent. Epitaxy semiconductorlayer 20-2 may also be formed of SiP, SiC, SiPC, SiGeB, or a III-Vcompound semiconductor such as InP, GaAs, AlAs, InAs, InAlAs, InGaAs, orthe like. Epitaxy semiconductor layer 20-2 may also be substantiallyfree from silicon, for example, with a silicon atomic percentage lowerthan about 1 percent.

In accordance with some embodiments of the present disclosure, theillustrated region is a p-type device region, in which a p-typetransistor such as a p-type FinFET is to be formed. Accordingly, epitaxysemiconductor layer 20-2 may be formed. In accordance with someembodiments of the present disclosure, the illustrated region is ann-type device region, in which an n-type transistor such as an n-typeFinFET is to be formed. Epitaxy layer 20-2 may extend into the p-typedevice region. If an n-type FinFET is formed, the respective deviceregion may not have the epitaxy layer 20-2 formed therein. The p-typedevice region and the n-type FinFET region may be on a same wafer and asame device die. Accordingly, a dashed line is drawn between substrateportions 20-1 and 20-2 to show that epitaxy layer 20-2 may or may notexist in the illustrated device region.

Pad layer 22 and mask layer 24 may be formed on semiconductor substrate20. Pad layer 22 may be a thin film formed of silicon oxide. Inaccordance with some embodiments of the present disclosure, pad oxidelayer 22 is formed in a thermal oxidation process, wherein a top surfacelayer of semiconductor substrate 20 is oxidized. Pad layer 22 acts as anadhesion layer between semiconductor substrate 20 and mask layer 24. Padlayer 22 may also act as an etch stop layer for etching mask layer 24.In accordance with some embodiments of the present disclosure, masklayer 24 is formed of silicon nitride, for example, using Low-PressureChemical Vapor Deposition (LPCVD). In accordance with other embodimentsof the present disclosure, mask layer 24 is formed through thermalnitridation of silicon, Plasma Enhanced Chemical Vapor Deposition(PECVD), or plasma anodic nitridation. Mask layer 24 is used as a hardmask during subsequent photolithography processes.

Referring to FIG. 2, mask layer 24 and pad layer 22 are etched, exposingunderlying semiconductor substrate 20. The exposed semiconductorsubstrate 20 is then etched, forming trenches 26. The respective processis illustrated as process 202 in the process flow shown in FIG. 18. Theportions of semiconductor substrate 20 between neighboring trenches 26are referred to as semiconductor strip 30 hereinafter. Portions oftrenches 26 may have the shape of strips (when viewed in the top view ofwafer 10) that are parallel to each other, and trenches 26 are closelylocated from each other. In accordance with some embodiments of thepresent disclosure, the aspect ratio (the ratio of depth to width) oftrenches 26 is greater than abut 7, and may be greater than about 10.Although one semiconductor strip 30 is illustrated, a plurality ofsemiconductor strips may be formed as being parallel to each other, withtrenches 26 separating the plurality of semiconductor strips from eachother. In accordance with some embodiments in which epitaxysemiconductor layer 20-2 is formed, the bottoms of trenches 26 are lowerthan the interface 23 between substrate portion 20-1 and epitaxysemiconductor layer 20-2. Interface 23 is also the bottom surface ofepitaxy semiconductor layer 20-2.

Referring to FIG. 3, silicon layer 32 is deposited in accordance withsome embodiments of the present disclosure. The respective process isillustrated as process 204 in the process flow shown in FIG. 18. Thedeposition may be performed through a conformal deposition method suchas Chemical Vapor Deposition (CVD). Silicon layer 32 may be free orsubstantially free from other elements such as germanium, carbon, or thelike. For example, the atomic percentage of silicon in silicon layer 32may be higher than about 95 percent. Silicon layer 32 may be formed as acrystalline silicon layer or a polysilicon layer, which may be achieved,for example, by adjusting the temperature and the growth rate in thedeposition process. Silicon layer 32 may have a thickness greater thanabout 10 Å, so that it may act as an effective nitrogen blocking layerin subsequent processes. The thickness of silicon layer 32 may be in therange between about 10 Å and about 20 Å.

When epitaxy layer 20-2 is formed in preceding steps, silicon layer 32is formed. In the region in which epitaxy layer 20-2 is not formed andan entirety of semiconductor strip 30 is formed of silicon, siliconlayer 32 may or may not be formed (since semiconductor strip 30 itselfis formed of silicon). In FIG. 3, silicon layer 32 is shown with dashedlines to indicate it may or may not be formed in accordance with variousembodiments.

Oxygen-containing dielectric layer (which may be an oxide layer) 34 isthen deposited on silicon layer 32. The respective process isillustrated as process 206 in the process flow shown in FIG. 18. Inaccordance with some embodiments of the present disclosure,oxygen-containing dielectric layer 34 is formed of silicon oxide (SiO₂).In accordance with other embodiments of the present disclosure,oxygen-containing dielectric layer 34 is formed of silicon oxy-carbide(SiOC), silicon oxy-nitride (SiON), silicon oxy-carbon-nitride (SiOCN),or the like. The formation method may include a conformal depositionmethod such as Atomic Layer Deposition (ALD), CVD, or the like. Thethickness of oxygen-containing dielectric layer 34 is selected to bethin enough so that nitrogen atoms can penetrate through it in thesubsequent anneal process, while still thick enough to function as abarrier of nitrogen atoms, so that nitrogen atoms may accumulate betweenoxygen-containing dielectric layer 34 and silicon layer 32 in subsequentprocesses. In accordance with some embodiments of the presentdisclosure, the thickness of oxygen-containing dielectric layer 34 is inthe range between about 15 Å and about 50 Å. When the thickness ofoxygen-containing dielectric layer 34 is out of this range,oxygen-containing dielectric layer 34 either does not allow thepenetration of nitrogen atoms, or is not able to keep nitrogen atoms toaccumulate between layers 32 and 34 to an intended thickness.

FIG. 4 illustrates the formation of (first) dielectric barrier layer 36between oxygen-containing dielectric layer 34 and silicon layer 32. Inaccordance with some embodiments of the present disclosure, theformation is achieved by thermally treating wafer 10 in an environmentcontaining ammonia (NH₃). In accordance with some embodiments of thepresent disclosure, the formation is achieved by thermally treatingwafer 10 in an environment containing N₂, N₂H₂, or the like. Theenvironment may be a process chamber. The respective process isillustrated as process 208 in the process flow shown in FIG. 18. Inaccordance with some embodiments of the present disclosure, the pressureof NH₃ is in the range between about 0.5 torr and about 10 torr. Thetemperature of the thermal treatment may be in the range between about500° C. and about 700° C., and the treatment time may be in the rangebetween about 20 minutes and about 40 minutes. In accordance with otherembodiments of the present disclosure, the temperature of the thermaltreatment is in the range between about 900° C. and about 1,100° C., andthe treatment time may be in the range between about 1 millisecond andabout 5 milliseconds. During the thermal treatment, no plasma isgenerated, and hence the thermal treatment is performed in a plasma-freeenvironment.

As a result of the treatment in the NH₃-containing environment, nitrogenatoms in NH₃ penetrate through oxygen-containing dielectric layer 34.The oxygen in oxygen-containing dielectric layer 34 makesoxygen-containing dielectric layer 34 permeable for nitrogen atoms topenetrate through. After the nitrogen atoms in NH₃ penetrate throughoxygen-containing dielectric layer 34, the penetrated nitrogen atoms areblocked by silicon layer 32, which is an effective blocking layer fornitrogen. Nitrogen atoms are thus accumulated between oxygen-containingdielectric layer 34 and silicon layer 32 to form dielectric barrierlayer 36. In accordance with some embodiments of the present disclosure,silicon-and-nitrogen-containing dielectric barrier layer 36 comprisessilicon, nitride, and possibly carbon and oxygen. Dielectric barrierlayer 36 may be formed as a conformal layer or a substantially conformallayer where oxygen-containing dielectric layer 34 contacts silicon layer32 both exist. The nitrogen atomic percentage in dielectric barrierlayer 36 may be greater than about 30 percent, and may be in the rangebetween about 40 percent and about 70 percent. Dielectric barrier layer36 may include a small amount of hydrogen, oxygen, and carbon (forexample, less than about 20 atomic percent in combination) therein.Dielectric barrier layer may have a middle portion with a peak nitrogenatomic percentage, and opposite side portions on the opposite sides ofthe middle portion, with the opposite portions having gradually reducednitrogen atomic percentages than the middle portion. Accordingly, in adirection pointing from the middle portion toward silicon layer 32, theatomic percentages of nitrogen and oxygen gradually reduces, and theatomic percentage of silicon gradually increases, in dielectric barrierlayer 36. In a direction pointing from the middle portion (the peak ofnitrogen) toward oxygen-containing dielectric layer 34, the atomicpercentage of oxygen gradually increases, and the atomic percentage ofsilicon and nitrogen gradually reduce, in dielectric barrier layer 36.In accordance with some embodiments, the carbon percentages in layers32, 34, and 36 are close to each other.

It is appreciated that the thickness of dielectric barrier layer 36 isaffected by various factors such as the composition and the thickness ofoxygen-containing dielectric layer 34. For example, a thickeroxygen-containing dielectric layer 34 may result in a thicker dielectricbarrier layer 36. However, a thicker dielectric barrier layer 36 isformed by longer treatment time and/or higher treatment temperature. Inaccordance with some embodiments of the present disclosure, thethickness of dielectric barrier layer 36 is in the range between about 1Å and about 10 Å. Accordingly, to allow the thickness of dielectricbarrier layer 36 to fall into the desirable range, an appropriatethickness of oxygen-containing dielectric layer 34 is adopted.Furthermore, experiments may be performed on sample wafers usingdifferent combinations of process conditions/factors (such as thethickness of layer 34, the anneal time and anneal temperature, etc.) tofind a set of process factors (and conditions), so that the thickness ofthe resulting dielectric barrier layer 36 may fall into the desirable(target) range. The desirable thickness of dielectric barrier layer 36and its effect are also discussed in subsequent paragraphs. Thethickness of dielectric barrier layer 36 may be smaller than both thethickness of silicon layer 32 and the thickness of oxygen-containingdielectric layer 34. Also, the thickness of silicon layer 32 may besmaller than the thickness of oxygen-containing dielectric layer 34.

In the initial stage of the treatment, more nitrogen atoms are diffusedtoward silicon layer 32 from the NH₃-containing environment. After thenitrogen atoms are accumulated, more and more nitrogen atoms arediffused back toward the NH₃-containing environment from the accumulateddielectric barrier layer 36. When the treatment is long enough,equilibrium is established, and the thickness of dielectric barrierlayer 36 is maintained substantially unchanged even if the anneal isprolonged.

Dielectric material 40 is then formed to fill the remaining portions oftrenches 26, and then planarized, resulting in the structure shown inFIG. 5. The respective process is illustrated as process 210 in theprocess flow shown in FIG. 18. The formation method of dielectricmaterial 40 may be selected from Flowable Chemical Vapor Deposition(FCVD), spin-on coating, CVD, ALD, High-Density Plasma Chemical VaporDeposition (HDPCVD), Low Pressure CVD (LPCVD), and the like.

In accordance with some embodiments in which FCVD is used, a silicon-and nitrogen-containing precursor (for example, trisilylamine (TSA) ordisilylamine (DSA)) is used, and hence the resulting dielectric material40 is flowable. In accordance with alternative embodiments of thepresent disclosure, the flowable dielectric material 40 is formed usingan alkylamino silane based precursor. During the deposition, plasma isturned on to activate the gaseous precursors for forming the flowableoxide. After dielectric material 40 is formed, an anneal/curing processis performed, which converts flowable dielectric material 40 into asolid dielectric material. The solidified dielectric material is alsoreferred to as dielectric material 40.

In accordance with some exemplary embodiments of the present disclosure,the anneal is performed in an oxygen-containing environment. Theannealing temperature may be higher than about 200° C., for example, ina temperature range between about 200° C. and about 700° C. During thethermal treatment, an oxygen-containing process gas is conducted intothe process chamber in which wafer 10 is placed. The oxygen-containingprocess gas may include oxygen (O₂), ozone (O₃), or combinationsthereof. Steam (H₂O) may also be used. As a result of the thermaltreatment, dielectric material 40 is cured and solidified. The resultingdielectric material 40 may be an oxide. A planarization process such asa Chemical Mechanical Polish (CMP) process or a mechanical grindingprocess may be performed to level the top surface of dielectric material40. In the planarization process, hard mask 24 may be used as a stoplayer.

In the curing of dielectric material 40, oxygen may migrate fromdielectric material 40 to semiconductor strip 30, causing the outerportions of semiconductor strip 30 to be oxidized. This results in theundesirable thinning of semiconductor strip 30. In accordance with someembodiments of the present disclosure, dielectric barrier layer 36 isformed as a blocking layer of oxygen, which prevents oxygen fromreaching semiconductor strip 30. A thicker dielectric barrier layer 36is more effective in blocking oxygen. On the other hand, a thickerdielectric barrier layer 36 attracts more charges in semiconductor strip30 and the underlying portion of substrate 20, thus results in morecharges to be accumulated in silicon layer 32 and its adjacent portionsof substrate 20 and semiconductor strip 30. The charge-accumulated layerforms a channel for leakage current. Accordingly, dielectric barrierlayer 36 cannot be too thick. To balance the effect of blocking oxygenwithout resulting in the adverse increase in leakage current, thethickness of dielectric barrier layer 36 may be selected to be in therange between about 1 Å and about 10 Å.

In accordance with some embodiments of the present disclosure, thecombination of oxygen-containing dielectric layer 34 and dielectricbarrier layer 36 show good results in both the reduction of leakage andoxidation prevention (due to dielectric barrier layer 36). The goodresults are due to the existence of oxygen-containing dielectric layer34 that does not attract charges, and further due to the limitedthickness of dielectric barrier layer 36.

Next, as shown in FIG. 6, the dielectric regions including silicon layer32, dielectric barrier layer 36, and oxygen-containing dielectric layer34 are recessed, and the resulting dielectric regions are referred to asdielectric regions 42. The respective process is illustrated as process212 in the process flow shown in FIG. 18. Throughout the description,dielectric regions 42 are alternatively referred to isolation regions 42or Shallow Trench Isolation (STI) regions 42. The portion ofsemiconductor strip 30 (and the portion of silicon layer 32) higher thanSTI regions 42 are referred to as protruding (semiconductor) fin 44. Inaccordance with some embodiments of the present disclosure, the topsurfaces of STI regions 42 are higher than the bottom surface 23 ofepitaxy layer 20-2 (if formed). The recessing of the dielectric regionsmay be performed using a dry etch process, in which HF₃ and NH₃ are usedas the etching gases. In accordance with alternative embodiments of thepresent disclosure, the recessing of the dielectric regions is performedusing a wet etch process. The etching chemical may include HF solution,for example.

In above-illustrated embodiments, semiconductor fins may be formed byany suitable method. For example, the semiconductor fins may bepatterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers, or mandrels, may then be usedto pattern the fins.

Referring to FIG. 7, dummy gate stacks 46 are formed to cross protrudingfin 44. The respective process is illustrated as process 214 in theprocess flow shown in FIG. 18. Dummy gate stacks 46 may include dummygate dielectrics 48 and dummy gate electrodes 50 over dummy gatedielectrics 48. Dummy gate dielectrics 48 may be formed of silicon oxideor other dielectric materials. Dummy gate electrodes 50 may be formed,for example, using polysilicon or amorphous silicon, and other materialsmay also be used. Each of dummy gate stacks 46 may also include one (ora plurality of) hard mask layer 52 over dummy gate electrode 50. Hardmask layers 52 may be formed of silicon nitride, silicon oxide, siliconcarbo-nitride, or multi-layers thereof. Dummy gate stacks 46 may crossover a single one or a plurality of protruding fin 44 and/or STI regions42. Dummy gate stacks 46 also have lengthwise directions perpendicularto the lengthwise directions of protruding fin 44. The formation ofdummy gate stacks 46 includes depositing a dummy gate dielectric layer,depositing a gate electrode layer over the dummy gate dielectric layer,depositing a hard mask layer, and patterning the stack layers to formdummy gate stacks 46.

Next, referring to FIG. 8, gate spacers 54 are formed on the sidewallsof dummy gate stacks 46. The respective process is illustrated asprocess 216 in the process flow shown in FIG. 18. The formation of gatespacers 54 may include depositing a blanket dielectric layer, andperforming an anisotropic etch to remove the horizontal portions of thedielectric layer, leaving gate spacers 54 to be on the sidewalls ofdummy gate stacks 46. In accordance with some embodiments of the presentdisclosure, gate spacers 54 are formed of an oxygen-containingdielectric material (an oxide) such as SiO₂, SiOC, SiOCN, or the like.In accordance with some embodiments of the present disclosure, gatespacers 54 may also include a non-oxide dielectric material such assilicon nitride.

Referring to FIG. 9, a thermal treatment is performed to formsilicon-and-nitrogen-containing layer 56. The respective process isillustrated as process 218 in the process flow shown in FIG. 18. Thetreatment is similar to the treatment process for forming dielectricbarrier layer 36. The process details may be essentially the same as theprocess details for forming dielectric barrier layer 36, and hence arenot repeated herein. The details (such as the composition, thethickness, etc.) of the resulting silicon-and-nitrogen-containing layer56 may be similar to that of silicon nitride layers 36. Throughout thedescription, silicon-and-nitrogen-containing layer 56 is alternativelyreferred to as silicon nitride layer 56 although it may include otherelements such as carbon, oxygen, or the like. In accordance with someembodiments in which dummy gate electrodes 50 are formed of polysiliconor amorphous silicon, dummy gate electrodes 50 act as the blocking layerfor blocking nitrogen atoms, and hence no additional silicon layer isformed for the purpose of blocking nitrogen atoms. In accordance withalternative embodiments in which gate electrodes 50 are formed of anon-silicon material (such as amorphous carbon), an additional siliconlayer (not shown) is formed on the sidewalls of dummy gate stacks 46before the formation of gate spacers 54. Accordingly, silicon nitridelayer 56 will be formed between the additional silicon layer and gatespacer 54. In subsequent paragraphs, silicon nitride layers 56 and gatespacers 54 are in combination referred to as gate spacers 58.

In accordance with alternative embodiments of the present disclosure,the thermal treatment for forming silicon nitride layer 56 is skipped,and hence silicon nitride layer 56 is not formed.

In a subsequent step, an etching process (referred to as fin recessinghereinafter) is performed to etch the portions of protruding fin 44 thatare not covered by dummy gate stacks 46 and gate spacers 58, resultingin the structure shown in FIG. 10. The respective process is illustratedas process 220 in the process flow shown in FIG. 18. The recessing ofprotruding fin 44 may be performed through an anisotropic etchingprocess, and hence the portions of protruding fin 44 directly underlyingdummy gate stacks 46 and gate spacers 58 are protected, and are notetched. The top surfaces of the recessed semiconductor strip 30 may belower than the top surfaces 42A of STI regions 42 in accordance withsome embodiments. Recesses 60 are accordingly formed between STI regions42. Recesses 60 are located on the opposite sides of dummy gate stacks46. In the recessing, the portions of silicon layer 32 higher than thebottom surfaces 60A of recesses 60 are also etched, hence the sidewallsof silicon nitride layers 36 are exposed. The bottom surfaces 60A mayalso be higher than, level with, or lower than, the interface 23.Accordingly, there may be, or may not be, remaining portions of epitaxysemiconductor layer 20-2 directly underlying recesses 60.

Next, epitaxy regions (source/drain regions) 62 are formed byselectively growing a semiconductor material from recesses 60, resultingin the structure in FIG. 11. The respective process is illustrated asprocess 222 in the process flow shown in FIG. 18. In accordance withsome embodiments of the present disclosure, epitaxy regions 62 includesilicon germanium, silicon, or silicon carbon. Depending on whether theresulting FinFET is a p-type FinFET or an n-type FinFET, a p-type or ann-type impurity may be in-situ doped with the proceeding of the epitaxy.For example, when the resulting FinFET is a p-type FinFET, silicongermanium boron (SiGeB), GeB, or the like may be grown. Conversely, whenthe resulting FinFET is an n-type FinFET, silicon phosphorous (SiP),silicon carbon phosphorous (SiCP), or the like, may be grown. Inaccordance with alternative embodiments of the present disclosure,epitaxy regions 62 are formed of a III-V compound semiconductor such asGaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlAs, AlP, GaP, combinationsthereof, or multi-layers thereof. After epitaxy regions 62 fully fillrecesses 60, epitaxy regions 62 start expanding horizontally, and facetsmay be formed.

After the epitaxy step, epitaxy regions 62 may be further implanted witha p-type or an n-type impurity to form source and drain regions, whichare also denoted using reference numeral 62. In accordance withalternative embodiments of the present disclosure, the implantationprocess is skipped when epitaxy regions 62 are in-situ doped with thep-type or n-type impurity during the epitaxy.

In accordance with alternative embodiments of the present disclosure,instead of recessing protruding fin 44 and re-growing source/drainregions 62, cladding source/drain regions are formed. In accordance withthese embodiments, the protruding fin 44 as shown in FIG. 9 is notrecessed, and epitaxy regions (not shown) are grown on protruding fin44. The material of the grown epitaxy regions may be similar to thematerial of the epitaxy semiconductor material 62 as shown in FIG. 11,depending on whether the resulting FinFET is a p-type or an n-typeFinFET. Accordingly, source/drain regions 62 include protruding fin 44and the epitaxy regions. An implantation may (or may not) be performedto implant an n-type impurity or a p-type impurity.

FIG. 12 illustrates a perspective view of the structure after theformation of Contact Etch Stop Layer (CESL) 66 and Inter-LayerDielectric (ILD) 68. The respective process is illustrated as process224 in the process flow shown in FIG. 18. CESL 66 may be formed ofsilicon nitride, silicon carbo-nitride, or the like. CESL 66 may beformed using a conformal deposition method such as ALD or CVD, forexample. ILD 68 may include a dielectric material formed using, forexample, FCVD, spin-on coating, CVD, or other deposition methods. ILD 68may also be formed of an oxygen-containing dielectric material, whichmay be a silicon-oxide based material such as a Tetra Ethyl OrthoSilicate (TEOS) oxide, a Plasma-Enhanced CVD (PECVD) oxide (SiO₂),Phospho-Silicate Glass (PSG), Boro-Silicate Glass (BSG), Boron-DopedPhospho-Silicate Glass (BPSG), or the like. A planarization process suchas a Chemical Mechanical Polish (CMP) process or a mechanical grindingprocess is performed to level the top surfaces of ILD 68, dummy gatestacks 46, and gate spacers 58 with each other. In the formation of ILD68, an anneal process may be adopted. In accordance with someembodiments of the present disclosure, silicon nitride layer 56 acts asthe blocking layer to block oxygen from penetrating through and reachingprotruding fin 44 (not visible in FIG. 12, refer to FIGS. 6 and 7).

Next, dummy gate stacks 46, which include hard mask layers 52, dummygate electrodes 50 and dummy gate dielectrics 48, are etched in one or aplurality of etching processes, resulting in trenches 70 to be formedbetween opposite portions of gate spacers 58, as shown in FIG. 13. Therespective process is illustrated as process 226 in the process flowshown in FIG. 18. The etching process may be performed using, forexample, dry etching. Plasma may also be turned on in the etchingprocess. The etching gases are selected based on the material to beetched. For example, when hard masks 36 include silicon nitride, theetching gas may include fluorine-containing process gases such asCF4/O2/N2, NF3/O2, SF6, or SF6/O2, or the like. Dummy gate electrodes 50may be etched using C2F6, CF4, SO2, the mixture of HBr, Cl2, and O2, orthe mixture of HBr, Cl 2, O2, and CF2 etc. Dummy gate dielectrics 48 maybe etched using the mixture of NF3 and NH3 or the mixture of HF and NH3.If silicon layers are formed on the sidewalls of dummy gate stacks 46,the silicon layers are also removed.

As a result of the etching of dummy gate stacks 46, silicon nitridelayers 56 are exposed to trenches 70. In the etching process, siliconnitride layers 56 may be thinned, for example, to a thickness in therange between about 1 Å and about 5 Å. In accordance with someembodiment in which the original silicon nitride layers 56 are very thinbefore the removal of dummy gate stacks 46, silicon nitride layers 56may also be removed as a result of the etching, and hence the sidewallsof gate spacers 54 are exposed to trenches 70. Silicon nitride layers56, with a high nitrogen percentage, are more resistant to the damagecaused by the plasma used in the etching of dummy gate stacks 46.

Next, referring to FIG. 14, (replacement) gate stacks 72 are formed,which include gate dielectrics 74 and gate electrodes 76. The respectiveprocess is illustrated as process 228 in the process flow shown in FIG.18. The formation of gate stacks 72 includes forming/depositing aplurality of layers, and then performing a planarization process such asa CMP process or a mechanical grinding process. Gate dielectrics 74extend into the trenches 70 (FIG. 13). In accordance with someembodiments of the present disclosure, gate dielectrics 74 includeInterfacial Layers (ILs) 78 (FIGS. 17A and 17B) as their lower parts.ILs 78 are formed on the exposed surfaces of protruding fin 44. ILs 78may include an oxide layer such as a silicon oxide layer, which isformed through the thermal oxidation of protruding fin 44, a chemicaloxidation process, or a deposition process. Gate dielectrics 74 may alsoinclude high-k dielectric layers 80 (FIG. 9B) formed over ILs 78. High-kdielectric layers 80 may include a high-k dielectric material such asHfO₂, ZrO₂, HfZrOx, HfSiOx, HfSiON, ZrSiOx, HfZrSiOx, Al₂O₃, HfAlOx,HfAlN, ZrAlOx, La₂O₃, TiO₂, Yb₂O₃, silicon nitride, or the like. Thedielectric constant (k-value) of the high-k dielectric material ishigher than 3.9, and may be higher than about 7.0. High-k dielectriclayers 80 are formed as conformal layers, and extend on the sidewalls ofprotruding fin 44 and the sidewalls of gate spacers 58. In accordancewith some embodiments of the present disclosure, high-k dielectriclayers 80 are formed using ALD or CVD.

Referring again to FIG. 14, gate electrodes 76 are formed on top of gatedielectrics 74, and fill the remaining portions of the trenches left bythe removed dummy gate stacks. The sub-layers in gate electrodes 76 arenot shown separately in FIG. 14, while in reality, the sub-layers aredistinguishable from each other due to the difference in theircompositions. The deposition of at least lower sub-layers may beperformed using conformal deposition methods such as ALD or CVD, so thatthe thickness of the vertical portions and the thickness of thehorizontal portions of gate electrodes 76 (of each of sub-layers) aresubstantially equal to each other

Gate electrodes 76 may include a plurality of layers including, and notlimited to, a Titanium Silicon Nitride (TSN) layer, a tantalum nitride(TaN) layer, a titanium nitride (TiN) layer, a titanium aluminum (TiAl)layer, an additional TiN and/or TaN layer, and a filling metal. Some ofthese layers define the work function of the respective FinFET.Furthermore, the metal layers of a p-type FinFET and the metal layers ofan n-type FinFET may be different from each other so that the workfunctions of the metal layers are suitable for the respective p-type orn-type FinFETs. The filling metal may include aluminum, copper, orcobalt.

Next, as shown in FIG. 15, hard masks 82 are formed. In accordance withsome embodiments of the present disclosure, the formation of hard masks82 includes recessing replacement gate stacks 72 through etching to formrecesses, filling a dielectric material into the recesses, andperforming a planarization to remove the excess portions of thedielectric material. The remaining portions of the dielectric materialare hard masks 82. In accordance with some embodiments of the presentdisclosure, hard masks 82 are formed of silicon nitride, siliconoxynitride, silicon oxy-carbide, silicon oxy-carbo-nitride, or the like.

FIG. 16 illustrates the subsequent steps for forming contact plugs.Contact openings are first formed by etching into ILD 68 and CESL 66 toreveal source/drain regions 62. Silicide regions 84 and source/draincontact plugs 86 are then formed to extend into ILD 68 and CESL 66. Therespective process is illustrated as process 230 in the process flowshown in FIG. 18. The top edges of silicon nitride layers 36 may be incontact with silicide regions 84 or in contact with source/drain contactplugs 86, depending on where silicide regions 84 extend. Alternatively,the top edges of silicon nitride layers 36 may be in contact withsource/drain regions 62.

In a subsequent step, as shown in FIG. 17A, etch stop layer 88 isformed, followed by the formation of ILD 90. FIG. 17A shows across-sectional view obtained from the same plane that contains line A-Ain FIG. 16. In accordance with some embodiments of the presentdisclosure, etch stop layer 88 is formed of SiN, SiCN, SiC, SiOCN, oranother dielectric material. The formation method may include PECVD,ALD, CVD, or the like. The material of ILD 90 may be selected from thesame candidate materials (and methods) for forming ILD 68, and ILDs 68and 90 may be formed of the same or different dielectric materials. Inaccordance with some embodiments of the present disclosure, ILD 90 isformed using PECVD, FCVD, spin-on coating, or the like, and may includesilicon oxide (SiO₂).

ILD 90 and etch stop layer 88 are etched to form openings. The etchingmay be performed using, for example, Reactive Ion Etch (RIE). Gatecontact plug 92 and source/drain contact plugs 94 are formed in theopenings to electrically connect to gate electrodes 76 and source/draincontact plugs 86, respectively. FinFET 96 is thus formed.

FIG. 17B illustrates a cross-sectional view of FinFET 96 obtained fromanother plane, which is the same plane that contains line B-B in FIG.16. FIG. 17B illustrates silicon nitride layers 34 and 56 relative toother features.

The embodiments of the present disclosure have some advantageousfeatures. By forming silicon nitride layers, the advantageous feature ofsilicon nitride layers for preventing oxygen from reaching and oxidizingfins is achieved. On the other hand, the silicon nitride layers are verythin, and hence do not result in the adverse increase in leakagecurrents. The thin silicon nitride layer and oxide layer in combinationthus form good oxidation-resistant and leakage-proof barriers.

In accordance with some embodiments of the present disclosure, a methodincludes forming a silicon layer on a wafer, forming an oxide layer incontact with the silicon layer, and, after the oxide layer is formed,annealing the wafer in an environment comprising ammonia (NH₃) to form adielectric barrier layer between, and in contact with, the silicon layerand the oxide layer. The dielectric barrier layer comprises silicon andnitrogen. In accordance with an embodiment, the forming the dielectricbarrier layer comprises annealing the wafer in an environment comprisingammonia (NH₃). In accordance with an embodiment, the annealing isperformed at a temperature in a range between about 500° C. and about700° C., with an annealing duration in a range between about 20 minutesand about 40 minutes. In accordance with an embodiment, the annealing isperformed at a temperature in a range between about 900° C. and about1,100° C., with an annealing duration in a range between about 1millisecond and about 5 milliseconds. In accordance with an embodiment,during the annealing, plasma is turned off. In accordance with anembodiment, the forming the dielectric barrier layer comprises:conducting nitrogen atoms to penetrate through the oxide layer, whereinthe nitrogen atoms are blocked by the silicon layer. In accordance withan embodiment, the forming the silicon layer comprises epitaxiallygrowing a crystalline silicon layer. In accordance with an embodiment,the forming the silicon layer comprises depositing a polysilicon layer.In accordance with an embodiment, the silicon layer is substantiallyfree from germanium, and the silicon layer is formed on agermanium-containing semiconductor region. In accordance with anembodiment, the silicon layer is a dummy gate electrode, and the methodfurther comprises removing the dummy gate electrode to expose thedielectric barrier layer.

In accordance with some embodiments of the present disclosure, a methodincludes etching a semiconductor substrate of a wafer to form trenches,wherein a semiconductor strip is located between the trenches;depositing a silicon layer extending on sidewalls of the semiconductorstrip; depositing an oxygen-containing dielectric layer on the siliconlayer; annealing the wafer in an environment comprising ammonia; formingisolation regions in the trenches; recessing the isolation regions,wherein a top portion of the semiconductor strip higher than topsurfaces of the recessed isolation regions form a semiconductor fin;forming a gate stack on the semiconductor fin; and forming source/drainregions based on the semiconductor fin, wherein the source/drain regionsare on opposite sides of the gate stack. In accordance with anembodiment, the annealing results in dielectric barrier layer to beformed between the silicon layer and the oxygen-containing dielectriclayer. In accordance with an embodiment, the annealing is performed at atemperature in a range between about 500° C. and about 700° C., with anannealing duration in a range between about 20 minutes and about 40minutes. In accordance with an embodiment, the annealing is performed ata temperature in a range between about 900° C. and about 1,100° C., withan annealing duration in a range between about 1 millisecond and about 5milliseconds. In an embodiment, the method further includes epitaxiallygrowing a germanium-containing semiconductor layer over a siliconsubstrate, wherein the germanium-containing semiconductor layer and thesilicon substrate in combination form the semiconductor substrate, andthe silicon layer is formed to contact a remaining portion of thegermanium-containing semiconductor layer in the semiconductor strip.

In accordance with some embodiments of the present disclosure, a deviceincludes a semiconductor substrate; an isolation region extending intothe semiconductor substrate, wherein the isolation region comprises: adielectric barrier layer; and an oxide layer on the dielectric barrierlayer; a semiconductor fin protruding higher than a top surface of thedielectric barrier layer; and a semiconductor strip overlapped by thesemiconductor fin, wherein the semiconductor strip is in contact withthe dielectric barrier layer. In accordance with an embodiment, thesemiconductor strip comprises: a lower portion, wherein the lowerportion comprises silicon and is free from germanium; and an upperportion comprising: an inner portion formed of a germanium-containingsemiconductor material; and an outer portion formed of silicon, and theouter portion is substantially free from germanium, and the outerportion contacts the dielectric barrier layer. In accordance with anembodiment, an interface between the lower portion and the upper portionis at an intermediate level between a top surface and a bottom surfaceof the isolation region. In accordance with an embodiment, thedielectric barrier layer has a thickness in a range between about 1 Åand about 10 Å. In accordance with an embodiment, the dielectric barrierlayer further comprises carbon and oxygen.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device comprising: a bulk semiconductorsubstrate; an isolation region extending into the bulk semiconductorsubstrate, wherein the isolation region comprises: a dielectric barrierlayer comprising silicon and nitrogen; and an oxide layer on thedielectric barrier layer; a semiconductor fin protruding higher than atop surface of the dielectric barrier layer; and a semiconductor stripoverlapped by the semiconductor fin, wherein the semiconductor strip isin contact with the dielectric barrier layer.
 2. The device of claim 1,wherein the semiconductor strip comprises: a lower portion, wherein thelower portion comprises silicon and is free from germanium; and an upperportion comprising: an inner portion formed of a germanium-containingsemiconductor material; and an outer portion formed of silicon, whereinthe outer portion has a lower germanium atomic percentage than the innerportion, and the outer portion contacts the dielectric barrier layer. 3.The device of claim 2, wherein an interface between the lower portionand the upper portion is at an intermediate level between a top surfaceand a bottom surface of the isolation region.
 4. The device of claim 1,wherein the dielectric barrier layer has a thickness in a range betweenabout 1 Å and about 10 Å.
 5. The device of claim 1, wherein thedielectric barrier layer further comprises carbon and oxygen.
 6. Thedevice of claim 1, wherein the dielectric barrier layer forms a firstinterface with the semiconductor strip, and a second interface with theoxide layer, and wherein the dielectric barrier layer has a peaknitrogen atomic percentage at an intermediate position between the firstinterface and the second interface.
 7. The device of claim 6, wherein ina first direction pointing from the intermediate position to thesemiconductor strip, atomic percentages of nitrogen and oxygen in thedielectric barrier layer gradually reduce, and atomic percentages ofsilicon in the dielectric barrier layer gradually increase.
 8. Thedevice of claim 7, wherein in a second direction pointing from theintermediate position to the oxide layer, atomic percentages of oxygenin the dielectric barrier layer gradually increase, and atomicpercentages of silicon and nitrogen in the dielectric barrier layergradually reduce.
 9. The device of claim 1, wherein the dielectricbarrier layer comprises: a vertical portion contacting the semiconductorstrip; and a horizontal portion connecting to a bottom end of thevertical portion.
 10. The device of claim 1 further comprising: a gatestack on sidewalls and a top surface of the semiconductor fin; and agate spacer comprising: an additional oxide layer; and an additionaldielectric barrier layer between the gate stack and the additional oxidelayer, wherein the additional dielectric barrier layer has a highernitrogen atomic percentage than the additional oxide layer.
 11. A devicecomprising: a semiconductor substrate comprising a bulk portion; adielectric isolation region over the bulk portion of the semiconductorsubstrate, wherein the dielectric isolation region comprises: asilicon-and-nitrogen-containing dielectric barrier; asilicon-oxide-based dielectric layer over and contacting thesilicon-and-nitrogen-containing dielectric barrier, wherein thesilicon-oxide-based dielectric layer comprises a bottom portion and asidewall portion connecting to an end of the bottom portion; and adielectric region over the bottom portion of the silicon-oxide-baseddielectric layer, wherein the sidewall portion of thesilicon-oxide-based dielectric layer contacts an additional sidewall ofthe dielectric region; and a source/drain region physically contactingthe silicon-and-nitrogen-containing dielectric barrier.
 12. The deviceof claim 11, wherein the silicon-and-nitrogen-containing dielectricbarrier further comprises carbon.
 13. The device of claim 11, whereinthe silicon-and-nitrogen-containing dielectric barrier further comprisesoxygen.
 14. The device of claim 11 further comprising a gate stack on aside of the source/drain region, wherein both of thesilicon-and-nitrogen-containing dielectric barrier and thesilicon-oxide-based dielectric layer extends directly underlying thegate stack.
 15. The device of claim 11, wherein thesilicon-and-nitrogen-containing dielectric barrier has a thickness in arange between about 1 Å and about 10 Å.
 16. The device of claim 11,wherein the silicon-oxide-based dielectric layer is thicker than thesilicon-and-nitrogen-containing dielectric barrier.
 17. The device ofclaim 11, wherein the semiconductor substrate further comprises a stripportion overlapped by the source/drain region, wherein the strip portioncomprises: a silicon germanium inner portion; and a silicon outer layerbetween, and in contact with both of, the silicon-oxide-based dielectriclayer and the silicon germanium inner portion.
 18. A device comprising:a bulk semiconductor substrate; a dielectric isolation region over thebulk semiconductor substrate; a semiconductor strip protruding higherthan the bulk semiconductor substrate, wherein the semiconductor stripcomprises: a silicon germanium inner portion; and a silicon outer layerbetween, and in contact with both of, the dielectric isolation regionand the silicon germanium inner portion; and a source/drain regionoverlapping and contacting both of the silicon germanium inner portionand the silicon outer layer.
 19. The device of claim 18, wherein thesemiconductor strip further comprises a silicon region directlyunderlying the silicon germanium inner portion, wherein the siliconregion further contacts the silicon outer layer.
 20. The device of claim18, wherein the dielectric isolation region comprises: a silicon oxidelayer; and a silicon-and-nitrogen-containing dielectric barrier betweenthe silicon oxide layer and the semiconductor strip.